1. Field of the Invention
The present invention relates to a flat belt transmission comprising a flat pulley and a flat belt reeved around to the pulley and capable of transmitting a high load.
2. Prior Art
Heretofore, it has been well-known that in a flat belt transmission for transmitting a load by a flat belt and a flat pulley, as shown in FIG. 14, a crown 12 should be formed on the outer periphery of a pulley 11, which projects upwardly in the center of the pulley 11 in the width direction thereof in order to improve the tension distribution in the core 10a of a flat belt 10 and to prevent a snaking movement of the belt 10 (for example, as disclosed in Japanese Utility Model Laid Open Gazette No. 50-121147, Japanese Utility Model Laid Open Gazette No. 58-189844 and Japanese Utility Model Laid Open Gazette No. 62-2514).
In such a flat belt transmission, it has been conventionally throught that the shaft load (the load applied to the rotation shaft of one pulley 11 in a direction increasing the distance between shafts of the transmission pulleys) can be increased by increasing widths of the belt 10 and the pulley 11 with the result that transmission capacity can be increased and a high load can be transmitted. However, according to an experiment made by the inventors of the present invention, it was found that it was difficult to effectively transmit a high load even if the belt width was increased.
More specifically, the inventors of the present invention made an experiment in search of the relation between the width of the belt 10 and transmission capacity. According to the result of this experiment, as shown in FIG. 13, although the transmission capacity was increased in accordance with an increase in the belt width within the relatively narrow range of 12.7 mm-25.4 mm, the transmission capacity was not further increased even if the belt width was increased to 45 mm, at which width the capacity was almost the same as that when the width was 25.4 mm ("transmission capacity" on the ordinate of the graph shown in FIG. 13 is relatively shown by representing a predetermined value as a reference). From the result of the experiment, it was found that there was a limit to the amount the belt width could be increased and still improve transmission capacity in the flat belt transmission. In addition, the above experiment was made under the condition that the shaft load was 200 kg, the pair of flat pulleys 11 around which the belt 10 was reeved were 100 mm in diameter, the crown 12 of the pulley 11 had a 450 mm radius and the pulley 11 on the driving side was rotated at 2600 rpm. The slip ratio between the pulley 11 and the belt 10 was 1%-3%.
In order to find the reason for this, the bearing stress (the stress acting between the belt 10 and the pulley 11) generated in the belt 10 was measured and it was found that the bearing stress was not uniformly distributed. It was determined that the reason why transmission capacity was not increased in accordance with the increase in the belt width was due to a non-uniform distribution characteristic of the bearing stress generated in the belt 10. More specifically, because the crown 12 was convex at the center in the width direction of the pulley 11, it was found that the bearing stress of the belt 10 concentrated or peaked at the center thereof corresponding to the crown 12 as shown in FIG. 3(a). Therefore, as shown in FIG. 3(b), when the width of the belt 10 is increased, a portion which does not contribute to load transmission because the bearing stress is zero, is generated at both ends in the width direction of the belt 10 due to the peaking of the bearing stress in the center. Therefore, the width of contact of the belt 10 with the pulley 11 is not increased even if the belt width is further increased. Furthermore, a maximum bearing stress .delta.max at a part corresponding to the crown 12 of the pulley 11 is increased in accordance with an increase in the belt width and the shearing stress on the belt 10 is increased at the time of transmission, which exceeds the capacity of the belt, causing creeping movement and destruction of the belt 10, with the result that transmission capacity can not be effectively increased.
However, if there is no crown 12 on the outer periphery of the pulley 11, tension generated in the belt 10 is not uniform and snaking movement of the belt 10 is generated, causing the belt 10 to be destroyed at an early stage of transmission.